Serial line printers that incorporate a moving printhead carrier generally use some type of linear position encoder to provide closed-loop position control for the carrier. It is often desirable to print individual pixels or pels at a resolution greater than the fundamental resolution of the encoder. One conventional technique uses a quadrature signal developed from the base encoder to produce a four-times increase in resolution. While this approach is sufficient to provide high-resolution position information, it lacks the necessary accuracy to generate high-resolution fire pulse timing for the printhead. Typical tolerance values for a quadrature sensor are .+-.10 degrees on channel phase, and .+-.15% on duty cycle. These encoder output tolerances force the system designer to use the linear encoder fundamental signal as the base frequency for fire pulse generation.
The derivation of print fire pulse timing from carrier velocity measurements is subject to error from sudden changes in velocity over the distance of the encoder fundamental signal's period. For purposes of print fire pulse derivation, a single period of history of the quadrature signal must be used to provide pulses for the next encoder period. This technique is reliable as long as high frequency perturbations in the velocity do not result in the period being reduced or increased by a significant amount.
If the encoder, for example, produces 150 pulses per inch and the maximum desired print registration or resolution is 1200 pixels (or pels) per inch, the controlling logic must generate eight evenly spaced intervals within the 1/150-inch pulse window as the first step in developing the print fire timing. Individual printing elements, represented by ink jet nozzles in an array, are conventionally staggered within a single pel spacing to minimize the number of elements (i.e., nozzles) that must simultaneously fire. This reduces power requirements and aids in thermal management and fluid dynamics for the ink jet nozzles.
As a result of this nozzle stagger, each of the eight single pel print times must be subdivided into a much larger number of equal time intervals to segregate the specific elements according to their physical locations on the ink jet printhead. Thus, the nozzles are fired over a time lapse based upon the printhead carrier velocity which renders a vertical column on the media from the staggered nozzle array column on the printhead. Margin for error is built in to the system by the fact that some of the available time intervals during the individual pel print times are not needed to fire all of the nozzles.
In an example system of an eight-times expansion in precision to 1200 dpi, a collision between the fire pulses may occur if a subsequent 1/150-inch pulse window time period decreases by more than 12.5% from the previous 1/150-inch measurement. A very large acceleration is necessary to produce such a change in velocity for a typical carrier speed of twenty inches per second (ips), and most printers incorporate a motor system that does not have enough power to produce this acceleration. While this large error is necessary to cause collision between adjacent fire pulses, the drop placement accuracy is effected as soon as the increase in velocity brings the next encoder edge into the final fire window of the previous slice, which will then conflict with the first fire window of the next slice.
In addition, as the carrier moves across the print media, it can experience a rotational motion with respect to the carrier shaft. This rotational motion can cause the encoder strip sensor to see some additional acceleration in the direction of travel of the carrier, resulting from the vibration of the carrier. This new motion is translated into a high frequency disturbance in the velocity. Resultant fire pulse window calculations from such "false" encoder edges can result in pulse timing inaccuracy, and consequently defects in the print-out.
It would be a significant advantage to provide an ink jet printer that can "self-correct" the timing calculations based upon a practical range of carrier acceleration.